Methods of forming image sensor microlens structures

ABSTRACT

In one aspect, an image sensor is provided which includes an interlayer insulation film formed over a substrate including a light receiving device, a color filter formed over the interlayer insulation film, a protection film having a flat top face formed over the interlayer insulation film and the color filter, a buffer film having a convex top face formed over the protection film, and a microlens formed on the convex top face of the buffer film. The microlens has a refractive index which is greater than a refractive index of the buffer film and has a convex top face and a concave bottom face, where the concave bottom face of the microlens contacts the convex top face of the buffer film.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to semiconductor devices andmethod of forming the same, and in particular, the present inventionrelates to the microlens structure of image sensors and to methods offorming the same.

A claim of priority under 35 U.S.C. § 119 is made to Korean PatentApplication No. 2004-98665, filed on Nov. 29, 2004, the entire contentsof which are hereby incorporated by reference.

2. Description of the Related Art

The digital camera is a popular example of a device which utilizes imagesensor having an optical microlens and a light receiving element. Lightincident on the microlens is focused towards the light receiving elementwhich transforms the light into an electrical signal capable of imageprocessing.

The image sensor is usually composed of a large two-dimensional pixelarray, where each pixel includes an optical microlens and a lightreceiving element. In addition, transmission and signally devices areprovided to read out the images of the image sensor. Most image sensorscan be classified as either charge-coupled-device type image sensors(CCD images sensor) or complementary metal-oxide-semiconductor (CMOS)image sensors (CIS).

In both CIS and CCD image sensor, as the integration density continuesto increase, the spacing between adjacent pixels is becoming narrower.As a result, light incident at a target pixel tends to outflow toward anadjacent pixel. The problem is explained below in more detail withreference to FIG. 1.

Referring to FIG. 1, the conventional image sensor includes a lightshielding layer 104 located within interlayer insulation films 105 a and105 b. The light shielding layer 104 is intended to prevent light frombeing incident on a region 103 external the light receiving devices 102a, 102 b, and 102 c. Color filters 106 a, 106 b, and 106 c are arrangedover the light shielding layer 104. On the color filters 106 a˜106 c, aprotection film 107 having a flattened top surface is formed forplanarization and improving optical transmittance. Also, microlenses 108a, 108 b, and 108 c are arranged on the protection film 107 toconcentrate light thereon.

In general, the light receiving devices 102 a˜102 c are formed of photogates or photodiodes. The light shielding layer 104 is formed of metal.The color filters 106 a˜106 c are made of dyed photoresist materials.The microlenses 108 a˜108 c are mostly formed with using polymer resins,and the protection film 107 is usually made of silicon oxide that has arefractive index similar to that of the microlens.

In such a conventional image sensor, one side (upper surface) of themicrolens (e.g., one of 108 a˜108 c) is shaped with a convex curvature,whereas the other side (lower surface that faces the protection film107) is flat. In other words, the conventional microlens is configuredwith a unilateral convex top surface. The efractive index of theprotection film 107 is substantially equal to that of the microlenses108 a˜108 c. Thus, as shown in FIG. 1, a focal distance d_(F1) of themicrolens is shorter than a distance d_(A1) from the microlens to thelight receiving device. It means that a focus F of the microlens issettled on a position spaced well away from the light receiving device.As a result, the focusing efficiency is degraded and light rays arelikely to be incident on adjacent light receiving devices.

SUMMARY OF THE INVENTION

According to an aspect of the present invention, an image sensor isprovided which includes a microlens having a convex top face, and abuffer film having a convex top face, where the convex top face of thebuffer film contacting a concave bottom face of the microlens, and wherea refractive index of the buffer film is less than a refractive index ofthe microlens.

According to another aspect of the present invention, an image sensor isprovided which includes an interlayer insulation film formed over asubstrate including a light receiving device, a color filter formed overthe interlayer insulation film, a protection film having a flat top faceformed over the interlayer insulation film and the color filter, abuffer film having a convex top face formed over the protection film,and a microlens formed on the convex top face of the buffer film. Themicrolens has a refractive index which is greater than a refractiveindex of the buffer film and has a convex top face and a concave bottomface, where the concave bottom face of the microlens contacts the convextop face of the buffer film.

According to still another aspect of the present invention, a method offorming an image sensor is provided which includes forming a buffer filmover a substrate to have a flat top face, dry-etching the buffer film tocontour the buffer film into having a convex top face, and forming amicrolens on the convex top face of the buffer film, where the microlenshas a concave bottom face located on the convex top face of the bufferfilm.

According to yet another aspect of the present invention, a method offorming an image sensor is provided which includes forming an interlayerinsulation film over a substrate including light receiving devices,forming color filters over the interlayer insulation film and alignedover the light receiving devices, forming a buffer film over theinterlayer insulation film which covers the color filters and which hasconvex top faces aligned over the color filters, and forming microlenseson the convex top faces of the buffer film. The microlenses has arefractive index which is greater than a refractive index of the bufferfilm.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention, and are incorporated in and constitute apart of this specification. The drawings illustrate example embodimentsof the present invention and, together with the description, serve toexplain principles of the present invention. In the drawings:

FIG. 1 is a sectional diagram for using in explaining drawbacksencountered in a conventional image sensor;

FIG. 2 is a sectional diagram illustrating the structure of an imagesensor in accordance with an exemplary embodiment of the presentinvention; and

FIGS. 3 through 6 are sectional diagrams for use in explaining a methodof manufacturing the image sensor illustrated in FIG. 2 in accordancewith an exemplary embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred embodiments of the present invention will be described belowin detail with reference to the accompanying drawings. The presentinvention may, however, be embodied in different forms and should not beconstructed as limited to the embodiments set forth herein. Rather,these embodiments are provided so that this disclosure will be thoroughand complete, and will fully convey the scope of the invention to thoseskilled in the art. Like numerals refer to like elements throughout thedrawings and specification.

It will be understood that, although the terms for first, second, third,etc. may be used herein to describe various elements such as region,film, layer, etc., these elements should not be limited by these terms.These terms are only used to distinguish one element from another. Forexample, a first layer could be termed a second layer, and, similarly, asecond layer could be termed a first layer without departing from theteaching of this disclosure. In the drawings, the thicknesses of layersand regions may be exaggerated for clarity. It will also be understoodthat when a layer is referred to as being on another layer or substrate,it can be directly on the other layer or substrate, or interveninglayers may also be present.

Some embodiments described herein are directed to an image sensor whichis capable of enhancing focusing efficiency by increasing a focaldistance. The focal distance can be increased by using a microlenshaving double-sided curvatures. The focal distance may be furtherincreased by forming a buffer film under the microlens which has arefractive index that is lower than that of the microlens. In someembodiments, a concave bottom surface of the microlens is formed on aconvex top face of the buffer film.

FIG. 2 is a cross-sectional diagram illustrating the structure of animage sensor 100 having a microlens with double-sided curvatures inaccordance with an embodiment of the invention.

In FIG. 2, reference character “A” denotes a light transmission regionin which a light receiving device is positioned, and reference character“B” denotes a light shielding region in which metal connections,transistors, and shielding patterns are arranged. The light shieldingregion B may also include logic circuits. Although the image sensor 100will typically include a large array of light receiving devices, onlytwo such devices are shown in the FIG. 2 for simplicity of thedescription.

Referring to FIG. 2, the image sensor 100 of this example includes asubstrate 111 including light receiving devices 115 a and 115 b, aninterlayer insulation film 119 in which metal connections 117 areformed, color filters 121 a and 121 b, a protection film 123, a bufferfilm 125, and microlenses 127 a and 127 b.

The light receiving devices 115 a and 115 b are formed in the lighttransmission region A of the substrate 111 and are electrically isolatedfrom each other by field isolation regions 113. As suggested above, anarray of light receiving devices 115 a and 115 b are arranged on thesubstrate 111 in rows and columns. The light receiving devices 115 a and115 b may be configured as one or more of a variety of different lightreceiving device types, and examples include photodiodes,phototransistors, pinned photodiodes, photogates, and MOSFETs. In thecase of a photodiode, an N-type epitaxial silicon layer is grown in aP-type substrate and then N-type ion impurities are implanted into theN-type epitaxial silicon layer. Thereby, an N-type region of thephotodiode is formed therein. Subsequently, P-type ion impurities areimplanted into the surface of the N-type region to form a P-type regionof the photodiode. Signal charges are accumulated in the N-type regionof the photodiode. A deep P-type well can be formed as a barrier layerbetween the P-type substrate and N-type epitaxial silicon layer. Thedeep P-type well can be formed by implanting impurities after formingthe N-type epitaxial silicon layer.

Signal charges generated in the light receiving devices 115 a and 115 bare transferred by transistors (not shown) typically formed in the lightshielding region B and then read by a logic circuit (not shown).

The interlayer insulation film 119 is formed of silicon oxide which mayor may not be doped with impurities. Silicon oxide can be generallyclassified as either a low temperature oxide (LTO), a middle temperatureoxide (MTO), or a high temperature oxide (HTO), depending on fabricationtemperature. An “LTO” silicon oxide has a refractive index which islower than 1.5 and is formed at a temperature of about 100 to 150° C.Conventionally, an “interlayer insulation film” is formed of MTO or HTO,each of which has a higher refractive index (e.g., about 1.5 or more)than LTO and each of which is formed with different fabricationconditions than LTO. For example, MTO and HTO are formed at highertemperatures than that of LTO, and with different source gases andpressures than LTO.

On the interlayer insulation film 119, the color filters 121 a and 121 bare disposed. Color filters 121 a and 121 b are respectively alignedover the light receiving devices 115 a and 115 b. Likewise, microlenses127 a and 127 b are respectively aligned over the color filters 121 aand 121 b with the protection film 123 and the buffer film 125interposed there between.

The top faces of portions of the buffer film 125 are convex. The topfaces 127 s 1 of the microlenses 127 a and 127 b are convex, while thebottom faces 127 s 2 of the microlenses 127 a and 127 b are concave. Thecurvatures of the bottom faces 127 s 2 may be generally dependent on orcorrespond to the curvatures of the top faces 127 s 1. In other words,both sides of the microlens 127 a or 127 b define surface curvatures(convex and concave). As a result, it can be seen (by tracing theoptical path of incident light) that a focal distance d_(F2) is longerthan that of the conventional image sensor shown in FIG. 1. Thus, adistance d₂ between the focus F and the light receiving device in theimage sensor of the present embodiment can be made shorter than thedistance d₁ of the conventional image sensor shown in FIG. 1. Also, itis possible to make the total distance d_(A2) between the microlens andthe light receiving element longer in the present embodiment than thecorresponding distance d_(A1) of the conventional sensor shown inFIG. 1. These advantages result increasing the distance dF2 of the focusF by providing the bottom faces 127 s 2 of the microlens 127 a and 127 bwith concave surface curvatures. In order to further increase focaldistance d_(F2), the radius of curvature of the bottom surface 127 s 2can be made smaller than that of the top surface 127 s 1.

Still referring to FIG. 2, the buffer film 125 contacts the microlenses127 a and 127 b and has a refractive index which is lower than that ofthe microlenses 127 a and 127 b. In this manner, the focal distanced_(F2) may be increased even further, which enables the focus F of themicrolens 127 a or 127 b to be closely adjacent the light receivingdevice 115 a or 115 b. Comparably, in the conventional image sensorshown in FIG. 1, the refractive index of the protection film contactingto the microlenses is similar to that of the microlens.

The microlenses 127 a and 127 b may, for example, be formed of a polymerresin having a refractive index of about 1.5, or a photoresist material.The buffer film 125 may be formed of an LTO having a refractive index of1.43 to 1.45, or a polymer having a refractive index of 1.33 to 1.35.Here, the LTO film may be formed by a low-temperature chemical vapordeposition (LPCVD) process, e.g., at a low temperature of 100 to 150° C.in an atmosphere of silane and oxygen source gases. A polymer bufferfilm 125 may be formed at a temperature under 200° C.

The protection film 123 is interposed between the buffer film 125 andthe color filters 121 a and 121 b, and has a flattened top face. Theprotection film 123 may be formed of silicon oxide which may be the sameas the interlayer insulation film 119, having a refractive index higherthan that of the buffer film 125.

Preferably, the refractive index of the buffer film 125 is lower thanthose of the microlenses 127 a and 127 b and the protection film 123.

The protection film 123 may be formed of an LTO. Furthermore, theinterlayer insulation film 119 may be formed of an LTO. Such coincidenceof materials among the films 119, 123, and 125 may enhance focusingefficiency in the image sensor 100. In addition, the interlayerinsulation film 119A on the light receiving region A may be selectivelyformed of the LTO film. In forming the interlayer insulation film 119A,the interlayer insulation film 119 is first formed of a silicon oxideand then partially removed on the light receiving region A by means ofphotolithography. Next, an LTO film is deposited where the interlayerinsulation film 119 has been removed, resulting in the interlayerinsulation film 119A.

A method of forming the microlens structures of the image sensor 100shown in FIG. 2 will now be described with reference to FIGS. 3 through6. FIGS. 3 through 6 are presented here to explain an exemplary processof fabricating the microlenses only, while techniques for manufacturingof the underlying structures of the image sensor 100 will be wellunderstood by those skilled in the art from the foregoing description.

First, referring to FIG. 3, color filters 221 a and 221 b are formed ona substrate 211 in which light receiving devices, transistors, metalconnections, and interlayer insulation films have been already formed.The color filters 221 a and 221 b may be formed by well-known methods,e.g., they may be formed of a dyed photoresist.

A buffer film 224 is deposited covering the color filters 221 a and 221b. The buffer film 225 is made of a polymer or an LTO film. A polymerbuffer film 225 has a refractive index of 1.33 to 1.35 which is lowerthan that of the interlayer insulation film disposed under the bufferfilm 225. An LTO buffer film 225 has a refractive index of 1.43 to 1.45which is also lower than that of the interlayer insulation film.

Mask pattern 226 a˜226 c is then formed on the buffer film 225. Asillustrated, the mask pattern 226 a˜226 c includes openings which exposeportions of the buffer film 225 aligned over the color filters 221 a and221 b. The mask pattern 226 a˜226 c may be formed of a photoresist.

Although not shown, it may be preferable to form a flattened protectionfilm before depositing the buffer film 225. The flattened protectionfilm may be helpful in improving topology when later forming the maskpattern 226 a˜226 c on the buffer film 225.

Next, referring to FIG. 4, exposed portions of the buffer film 225 arepartially removed by way of a dry etching process. It is possible toadjust an etch rate of the buffer film 225, by using carbon-fluoridebased gas and oxygen gas, in accordance with a relative position of thebuffer film exposed by the mask pattern. The carbon-fluoride based gasmay contain C₄F₈ as an example. The etch rate can be controlled so as todecrease in a direction away from the edges of the mask pattern 226a˜226 c. Accordingly, parts of the buffer film contacting and closelyadjacent the edges of the mask pattern 226 a˜226 c are more rapidlyremoved than other parts spaced farther from the edges. As a result ofsuch dry etching techniques, as shown in FIG. 4, the resultant patternof the buffer film 225 is configured with convex top faces 225 s 2aligned over the color filters 221 a and 221 b.

In the dry etching process, the oxygen gas pressure can be adjusted tobe higher than that of the carbon-fluoride based gas, such that the topfaces of the buffer film 225 become more convex. The more convex the topfaces of the buffer film 225, the smaller the radius of curvature of thebuffer film 225. The smaller radius of curvature of the buffer film 225,the higher the focusing efficiency of the image sensor.

Referring now to FIG. 5, after removing the mask pattern 226 a˜226 c,microlens material is deposited on the buffer film 225, and microlenspatterns 228 a and 228 b are formed by exposing and patterningprocesses. The microlens material is selected so as to have an index ofrefraction which is greater than that of the buffer film 225. The bottomfaces of the microlens patterns 228 a and 228 b contact the convex topfaces 225 s 2 of the buffer film 225. As a result, each of the microlenspattern 228 a and 228 b has a concave bottom face which conforms to theshape of the convex top face 225 s 2 of the buffer film 225.

Next, referring to FIG. 6, the microlens patterns 228 a and 228 b aresubjected to heat treatment to cause re-flow of the microlens material.The re-flow process is conducted under the temperature that permitsmobility (i.e., flowing ability) of the microlens material. As a result,the top faces of the microlens patterns 228 a and 228 b are deformedinto microlenses 227 a and 227 b each having a convex top face 225 s 2.

Although the present invention has been described in connection with theembodiment of the present invention illustrated in the accompanyingdrawings, it is not limited thereto. It will be apparent to thoseskilled in the art that various substitution, modifications and changesmay be thereto without departing from the scope and spirit of theinvention.

1. A method of forming an image sensor comprising: forming a buffer filmfrom a low-temperature oxide (LTO) film over a substrate; forming a maskpattern comprising an opening on a top face of the buffer film anddry-etching the buffer film exposed by the opening, such that an etchrate is reduced in a direction away from edges of the opening, tocontour the buffer film into having a convex top face; and forming amicrolens on the convex top face of the buffer film, the microlenshaving a concave bottom face located on the convex top face of thebuffer film, wherein the buffer film has a refractive index less than arefractive index of the microlens.
 2. The method as set forth in claim1, wherein said dry-etching comprises: dry-etching the buffer film usingcarbon-fluoride based gas and oxygen gas.
 3. A method of forming animage sensor comprising: forming an interlayer insulation film over asubstrate including light receiving devices; forming color filters overthe interlayer insulation film, the color filters aligned over the lightreceiving devices; forming a buffer film over the interlayer insulationfilm and the color filters, the buffer film covering the color filtersand having convex top faces aligned over the color filters; and formingmicrolenses on the convex top faces of the buffer film, the microlenseshaving a refractive index which is greater than a refractive index ofthe buffer film, wherein forming the buffer film comprises: forming amask pattern over a flat top face of the buffer film, the mask patternincluding openings aligned over the color filters; dry-etching the flattop face of the buffer film exposed through the openings in the maskpattern, an etch rate of the dry-etching being reduced in a directionaway from edges of each opening of the mask pattern to produce theconvex top faces; and removing the mask pattern.
 4. The method as setforth in claim 3, wherein the dry-etching uses carbon-fluoride based gasand oxygen gas.
 5. The method as set forth in claim 3, wherein thebuffer film is formed of a polymer.
 6. The method as set forth in claim3, wherein the buffer film is formed of a low temperature oxidation(LTO) film.
 7. The method as set forth in claim 3, further comprising,before forming the buffer film, forming a protection film over theinterlayer insulation film and the color filters, the protection filmhaving a flat top surface and a refractive index which is greater thanthe refractive index of the buffer film.
 8. The method as set forth inclaim 7, wherein the buffer film is formed of one of a polymer or a lowtemperature oxidation (LTO) film, and the interlayer insulation film andthe protection film are formed of a silicon oxide film having arefractive index which is greater than the refractive index the bufferfilm.
 9. The method as set forth in claim 3, wherein forming themicrolenses comprises: forming a microlens pattern on the buffer filmhaving the convex top faces aligned over the color filters; andre-flowing the microlens pattern so as to conform to the convex topfaces of the buffer film.
 10. A method of forming an image sensorcomprising: forming a buffer film over a substrate to have a flat topface; dry-etching the buffer film to contour the buffer film into havinga convex top face; and forming a microlens on the convex top face of thebuffer film, the microlens having a concave bottom face located on theconvex top face of the buffer film, wherein dry-etching the buffer filmincludes forming a mask pattern on the flat top face of the buffer film,and dry-etching a portion of the buffer film exposed by the mask patternusing carbon-fluoride based gas and oxygen gas under a condition inwhich an etch rate is reduced in a direction away from an edge of themask pattern corresponding to the exposed portion of the buffer film.11. The method as set forth in claim 10, wherein forming the buffer filmcomprises providing a low-temperature oxide (LTO) film having arefractive index which is less than a refractive index of the microlens.12. The method as set forth in claim 10, wherein forming the buffer filmcomprises providing a polymer having a refractive index which is lessthan a refractive index of the microlens.
 13. The method as set forth inclaim 10, wherein forming the microlens comprises providing a polymerresin having a refractive index of about 1.5; and wherein forming thebuffer film comprises providing one of a low-temperature oxide (LTO)film having a refractive index in a range of 1.43 to 1.45 and a polymerhaving a refractive index in a range of 1.33 to 1.35.